The continually increasing combustion of fossil fuel, such as coal, natural gas and oil, during the last centuries has resulted in an increase in the concentration of CO2 in the atmosphere. The increasing concentration of CO2 has caused concern due to the greenhouse effect caused by CO2. The greenhouse effect is suspected already to have caused at least some of the changes in the climate that have been seen during the last decades, and is according to simulation models suspected to cause even more and potentially dramatic changes in the climate of planet earth.
This has caused a call for action from scientists, environmentalists and politicians throughout the world, to stabilize or even reduce the discharge of CO2 from combustion of fossil fuel into the atmosphere. A stabilization or even reduction of the discharge of CO2 into the atmosphere from combustion of fossil fuel may be achieved by capturing and safe depositing of CO2 from the exhaust gas from thermal power plants and other plants where fossil fuel is combusted.
The captured CO2 may be injected in sub terrain formations such as aquifers, oil wells for enhanced oil recovery or in depleted oil and gas wells for deposition. Tests indicate that CO2 remains in the sub terrain formation for thousands of years and is not released into the atmosphere.
Capturing of CO2 from a gas by means of absorption is well known and has been used for decades, e.g. for removal of CO2 (and other acid gases) from produced natural gas at gas fields. The absorbents used or suggested in the prior art have been different aqueous alkaline solutions, such as potassium carbonate and different amines, see e.g. U.S. Pat. Nos. 4,112,051, 4,397,660 and 5,061,465. Separation of CO2 from exhaust gas from thermal power plants by means of an amine solution, is known e.g. from U.S. Pat. No. 4,942,734.
Common for these CO2 capturing solutions is that the gas mixture to be separated is introduced countercurrent to the aqueous absorbent in an absorber column. The gas leaving the absorber column is CO2 depleted(or acid gas depleted), whereas the CO2 (or other acid gas) leaves the absorber column together with the absorbent. The absorbent is regenerated in the regenerator column and returned to the absorber column. Amine is regenerated by stripping the amine solution with steam in the regeneration column. The steam is generated in the reboiler at the base of the column.
The currently preferred absorbents are aqueous solutions of different amines. The commonly used amines are alkanol amines, such as e.g., monoethanol amine, diethanol amine, methyl-diethanolamine, piperazine, 2-amino-2-methyl propoanol, 2-metylaminoethanol, as well as other amines known by skilled man in the art. The absorption of CO2 to the amine absorbents is a reversible, exothermic reaction. Accordingly, heat has to be supplied to the regenerator column to reverse the absorption and release the CO2.
The vapor pressure and the solubility in water of the different amines varies considerably. Hence some amines are more likely to evaporate from aqueous absorbent solution and escape with the CO2 lean flue gas to the surroundings.
There are also large variations between different amines with respect to chemical stability towards oxidative and thermal degradation of the amine. Degradation products may be aldehydes, organic acids, and nitrogen containing compounds such as other amines, amides, nitrogen oxides and ammonia. Some of the degradation products are on ionic form or high molecular compounds and tends to accumulate in the solvent, while other volatile degradation products have limited solubility and follows the flue gas or CO2 product stream out of the capture plant system.
Recently a discussion on environmental or health hazards resulting from amines that are released from the plant together with the CO2 lean exhaust gas has been raised.
Amine absorbers for CO2 capture from flue gas are typically equipped with one or several water wash sections, where the main purpose is to reduce amine vapor slip to the atmosphere, see e.g. US2008159937. These water wash sections are normally either trays or packed bed sections with water recycling. The absorption of amine in the wash water section and corresponding amine concentration build-up requires a certain replacement of the wash water liquid, i.e. fresh water is added as make up and a bleed to the main amine solvent loop is established. The concentration of amine in the wash water must be maintained relatively low due to the amine vapour pressure (back pressure) from the wash water liquid. The vapour pressure is depending on the amine concentration and liquid temperature. Higher temperatures and higher amine concentrations cause higher amine vapour pressure from the liquid and thereby higher amine slip to air. One normally assumes equilibrium in the top of the water wash packed section with liquid recycling, meaning the gas will contain an amine concentration equivalent to the liquid vapour pressure, which again is dictated by the liquid temperature and amine concentration.
Adding excess amounts of fresh water to the water wash loop to dilute the solution, or operating with lower gas and liquid temperatures in order to reduce the amine vapour pressure, is challenging as the water balance over the entire capture plant must be maintained close to neutral. I.e. the sum of water coming into the system must balance the water leaving the system; otherwise the inventory will accumulate and must eventually be disposed off as waste. Both the use of excess amounts of water and operating at lower temperatures makes it difficult to achieve a near to zero amine vapor slip and this alternative operation will also have a significant increased cost impact.
Increased environmental awareness will generate a need for improved processes with minimum discharge to air of potential harmful compounds. It is therefore further anticipated that the authorities and industry is willing to incorporate such improvements as described herein.
The choice of an amine or a combination of amines for a plant for CO2 capture depends on different factors such as corrosive effect, possible or known environmental or health hazards, efficiency in CO2 capture, energy efficiency in the process, minimizing of amine degradation etc. If the amine slip into the atmosphere could be eliminated, amines that are harmful or not allowed due to environmental or health considerations related to amine slip, may be allowed in such processes.
There is therefore a need for improved methods and improved devices to eliminate or considerably reduce the slip of amines from CO2 sequestration plants.
Ammonia, if present as a possible degradation product of amine, is an alkaline compound and behaves similar to the amine itself, but is however significantly more volatile. Ammonia produced by degradation of amines in the solvent tends to escape the capture plant system via the emitted flue gas in the same rate as it is produced, since the solubility of ammonia in the solvent and water wash is very limited and equilibrium concentration will be quickly obtained. The concentration of ammonia in the emitted flue gas may however in some cases be above environmental regulation requirements, and a method for controlling ammonia emission from amine based CO2 capture plant is needed.